Resumen de Nanoestructuración de superficies de silicio mediante erosión iónica influencia de la incorporación simultánea de metales
The production of self-organized surface nanopatterns by ion beam sputtering (IBS) at low energies (<1 keV) has emerged in the last decade as a promising bottom-up nanostructuring tool. The technique enables large degree of control over the main pattern features with high throughput (it requires short process times and can be used over extended areas). To date, the studies of IBS pattern formation by different groups have contributed to a deepening of the knowledge of its underlying physical mechanisms. However, at the same time, they show clearly that there are still open issues concerning IBS surface patterning. One of these open issues refers to the problems usually found in patterning single-element semiconductor surfaces, particularly silicon, contrarily with the relatively easy patterning of compound semiconductors such as GaSb. Although the successful production of nanodot patterns by IBS on silicon surfaces has been reported, the lack of reproducibility has motivated some additional considerations such us the eventual need of surface impurities at the target surface, typically metals, to effectively induce such patterns.
The main issue of this thesis deals with the production of self-organized nanopatterns on silicon surfaces by IBS at low energy (< 1 keV) with simultaneous metal incorporation. The results reported here provide an important contribution to improve the knowledge on the physical mechanisms that govern the silicon nanopatterning by IBS in presence of metal impurities.
This thesis is divided in five main parts. In the introduction chapter it is given a broad overview of the different approaches to produce nanopatterns on surfaces, emphasizing that advantanges of IBS as an efficient technique for surface nanostructuring. It also includes a brief exposition of the state of the art of the recent advances in surface nanostructuring by IBS. The second chapter summarizes the main experimental procedures concerning the sputtering equipment as well as the characterization techniques, namely morphological, compositional, chemical and structural techniques used in this study. It should be stressed the relevance of atomic force microscopy (AFM) as a tool to image and characterize the resulting patterns.
The third chapter establishes the conditions for which nanohole and nanodot patterns are produced on Si (001) surfaces by 1 keV Ar+ ions impinging at normal incidence with an alternating cold cathode ion source (ACC-IS) and with simultaneous metal incorporation. After a systematic study, it is shown that nanohole patterns are produced within a narrow IBS window for low ion fluxes (< 100 ¿A/cm2) and relatively low ion fluencies (< 1018 iones/cm2) whereas nanodot morphologies are produced above this window. The quantitative determination of the metal content in the different patterns by Rutherford Backscattering spectrometry (RBS) shows that nanohole patterns are produced when the metal content is higher than in the case of nanodots. In order to gain further information on the metal incorporation, chemical analysis of the surface has been performed, which showed for the first time the relevant formation of metal silicides. In order to prove the generality of the correlation between the pattern morphology and surface metal content, experiments were performed with a more standard Kaufman-type source. These experiments led to results consistent with those found fot the ACC-IS, which, therefore, confirms the universality of the experimental findings of this work. Finally, further outlook and a discussion regarding the role of metal incorporation are also given.
The fourth chapter, focuses on the temporal evolution of self-organized nanodot patterns induced by 1 keV Ar+ on Si (001) surfaces with simultaneous metal incorporation for a wide range of ion fluxes. In this case, the experiments were performed with a Kaufman-type ion gun. It is concluded that the onset for pattern formation takes place earlier at higher ion flux, although the pattern morphological properties are similar for the same fluence values. These data have been successfully contrasted quantitatively with the theoretical simulations of a two-field continuum hydrodynamic model. The results can be explained once it is assumed the dominance of ion induced diffusion mechanisms, which is consistent with the employed experimental conditions. The second study addresses the influence of the target initial conditions (crystallinity and roughness) on the quantitative temporal evolution of the nanodot pattern morphological properties. AFM shows that similar nanodot patterns with short-range hexagonal order and dynamics can be obtained on either crystalline or amorphous silicon. This result reinforces the determinant role of the amorphous surface layer on the pattern formation. The influence of the initial target roughness on the nanodot pattern dynamics was studied by irradiating both smooth and rough amorphous silicon surfaces. Interestingly, initial rougher films are smoothed out before to the onset of pattern formation. However, after pattern stabilization, its morphology is equivalent on either rough or flat Si surfaces. Despite of the time required for initial smoothening, the pattern stabilization is attained at lower fluences on rougher targets.
Finally, the main conclusions of the thesis are compiled in the last chapter.
Also, this thesis includes two annexes. In the first one, there is a study of the damage caused by medium energy bombardment at normal incidence on Si (001) surfaces without simultaneous metal incorporation. This investigation was mainly carried out by spectroscopic ellipsometry (SE), RBS and transmission electron microscopy (TEM), showing that the ensemble of these techniques can be used for the quantitative analyses and accurate evaluation of the structural (thickness, density profile, implanted ions in-depth distribution) and optical (refractive index, optical band gap, etc.) properties of the damaged surface layer. RBS and SE measurements have allowed to observe a similar trend for the dependence of the amorphous density and refractive index with implantation energy and ion fluence. Particularly, it was found that the higher fluence, or the lower ion energy, the lower amorphous density. Finally, TEM analysis showed that in samples with subjected to high ion doses argon bubbles were formed in the irradiated volume. In the second annex, a brief account of the different theoretical models developed on IBS nanopatterning is given.
